This invention relates to mechanically-compensated compact zoom objectives having a large zoom ratio including a wide angle region and extending up to a telephoto region.
In a zoom objective lens system it is necessary to employ at least two lens components which move axially for varying the focal length of the entire system while maintaining a constant image plane. Recently there has been a desire to increase the zoom ratio of this type zoom objective while keeping the bulk and size to a minimum.
The zoom ratio of about two has been found practical in objectives for still cameras. Most of these objectives are constructed in the so-called short zoom form comprising two lens components of negative and positive powers as, for example, disclosed in U.S. Pat. Nos. 3,143,590, 3,848,969, 4,153,339, 4,142,779, and 4,147,410. Of these, there is known a lens system whose rear component having a positive power is divided into a plurality of parts to facilitate a remarkable increase in the zoom ratio. For example, U.S. Pat. No. 4,170,402 describes a system where the rear group comprises two positive lenses made movable independently of each other. In Japanese Patent Application Laid-Open No. Sho 53-34539, the rear group also is divided into three parts of positive, negative and positive powers of which the positive two are moved in unison with each other, while the intermediate or negative lens is moved in differential relation to the positive lenses. U.S. Pat. No. 3,771,853 describes the construction of a rear group of positive, negative and positive lenses which assume respectively different loci.
Aside from the direction of development of the art in the field of zoom objectives of the type consisting of negative and positive lens groups as described above, there is another direction of development toward an increase in the zoom ratio and the compactness based on the fundamental zoom lens design which has been widely accepted from. This development involves objectives comprising four lens components, namely, a focusing member, a variator, a compensator and a relay lens. For example, DOLS 2,833,308 describes a positive first lens group counting from the front, a negative second lens group and a positive third lens group constituting an entire system with exclusion of the relay lens. Here, the function of varying the focal length is imparted into the second and third lens groups, and both of the functions of focusing adjustment and image shift compensation are imparted into the first lens group. This achieves the objects of high zoom ratio and compactness.
To obtain a compact zoom objective of a high zoom ratio it is the common practice to strengthen the refractive power of the each individual movable lens group and narrow the air spacings between the successive lens groups. In this case, the following problems arise:
(1) The aberrational correction becomes difficult.
(2) The requirement for accuracy of adjustment in position of the movable lens group is extremely rigorous. If this is not fulfilled, the ideal optical performance cannot be obtained in completed objectives.
(3) No real solution can be found for the locus of movement of the compensator relative to the variator.